Prosocial behavior in competitive fish: the case of the archerfish

Humans are social creatures, demonstrate prosocial behaviors, and are sensitive to the actions and consequent payoff of others. This social sensitivity has also been found in many other species, though not in all. Research has suggested that prosocial tendencies are more pronounced in naturally cooperative species whose social structure requires a high level of interdependence and allomaternal care. The present study challenges this assumption by demonstrating, in a laboratory setting, that archerfish, competitive by nature, preferred targets rewarding both themselves and their tankmates, but only when the payoff was equal. With no tankmate on the other side of the partition, they exhibited no obvious preference. Finding evidence for prosocial behavior and negative responses to unequal distribution of reward to the advantage of the other fish suggests that in a competitive social environment, being prosocial may be the most adaptive strategy for personal survival, even if it benefits others as well.

Ten last trials of the social exp.

M(SD)
Analysis of all sessions Analysis from switch

Supplementary Note 2: Another replication of the control experiment.
A fourth fish participated in the control experiment but did not meet the criterion for accurate hits. When considering a wider radius, we found a pattern of results similar to that of the other control fish. That is, the fish showed no preference for any of the color targets: Analyses between the first phase of the first experiment and the second experiment. Since in the second phase there was also a reversal learning period, we compare only the first phase between the experiments. Fish 1,3, and 4 from the first social experiment also conducted the control experiment (see Table 1

Supplementary Note 4.
Analyses of all the data including the trials before the pre-defined reversal point for all fish- interval between appearances. After the location markers disappeared, two color targets appeared for 5500ms or until the fish responded. The targets appeared at the same location as the black location markers. After the targets disappeared, a blank interval screen was presented between trials for 5500ms. During this time, the fish received a food pellet regardless of which target it chose. The position of each target's color within the pair was counterbalanced.

Allocation of colors in each phase for each fish:
Note that for all fish in experiments one and two, the colors were switched in the second phase. In order to rule out that the fish cannot distinguish between the color green and blue please see note 6 and also please note that the colors used for Fish c were the same as it was for it in the social experiment (fish 4) in order to test whether the results of the other fish were dependent on the specific color properties employed.

Supplementary Note 6: Distinguishing between green and blue.
Despite evidence indicating that fish have color vision similar to that of humans [1,2] , we conducted a control experiment in order to rule out the possibility that the fish cannot distinguish between blue and green-the colors allocated to the control experiment.
In this experiment, one fish in a single tank was required to choose between a blue target, which resulted in receiving one food pellet, and a green target, which resulted in no food reward. We found a significant bias toward the blue target: t (9) = 4.09, p=0.003, d= 1.29,

Supplementary Note 7: Filtering rule.
Sessions in which the fish did not spit during more than 20 trials (half of the trials) were removed from the analysis. In total, in the first experiment, two sessions were removed for Fish 1 in the second phase and 11 sessions for Fish 4 in the first phase. In the second, control, experiment, one session was removed for Fish 2 in the first phase. In the third experiment, one session was removed for Fish 2. Six sessions were removed for Fish 3 in the first phase (the first-pro-social phase) and four sessions in the third experiment.

Supplementary Note 8: The pattern of results for each fish in the first session of the prosocial experiment.
In order to present the pattern of results in the first session, we calculated a moving average (with a window of five trials) for choosing the non-prosocial target. This analysis indicates two points: a) a more detailed pattern of preferences in the first introduction to the task, showing that there was exploration at the beginning with sampling of the non-social target that diminished as the session continued; b) an overall bias toward the prosocial target, mostly manifesting in the second half of the first session, indicating that it was not hard for the fish to learn the difference between the target outcomes. Fish c: phase 1 (V=52, p =1); phase 2 (V = 137, p=0.08 (1 food>2food))